1. Field of the Invention
The present invention relates to an antenna for wireless mobile communication systems and more particularly, to a retractable or extendable antenna for portable radio device, such as portable information terminals, portable or cellular phones, and so on, which is capable of improving impedance matching in the extended state and the retracted state.
2. Description of the Related Art
Conventionally, there are-several types of antennas for wireless mobile communication systems. An antenna of this type designed for portable radio device such as cellular phones typically has a linear whip element and a helical element fixed to one end of the whip element. The antenna is attached to the casing of the device in such a way that the whip element can be retracted into the casing and can be pulled out therefrom as necessary.
With the conventional portable radio devices having the antenna of this type, when the user operates the device, the whip antenna element is usually extended from the casing to reduce the antenna performance degradation due to the bad effect caused by the user himself or herself (ie., a human body). On the other hand, when the user does not operate the device, the whip element is retracted into the casing to facilitate carrying of the device.
When the antenna is pulled out from the casing, the whip and helical elements are located outside the casing. In this extended state, only the whip element is active and provides the desired antenna operation. On the other hand, when the antenna is pushed into the casing, the whip element is retracted into the casing while the helical element is located outside. In this retracted state, only the helical element is active and provides the desired antenna operation.
An example of the conventional antenna structures of this type is disclosed in the Japanese Non-Examined Patent Publication No. 9-186519 published in 1997 (which corresponds to the Japanese Patent No. 2,692,670 issued in 1999)
FIGS. 1A and 1B show schematically the structure of a prior-art antenna 101 of this type.
The antenna 101, which is mounted slidably on a casing 120, comprises a conductive, linear whip element 106 and a conductive helical (i.e., coil-shaped) element 117.
The whip element 106 is covered with a dielectric protection film 107. A conductive stopper 109 is fixed to the bottom end of the element 106. The stopper 109 prevents the element 106 from being detached from the casing 120 and serves to feed electric power to the element 106 in the extended state. A feeder or feeding part 104 is fixed to the top end of the element 106 by way of a dielectric separator 105. The feeder 104 serves to feed electric power to the element 106 when the element 106 is retracted into the casing 120. The separator 105 serves to separate electrically the helical element 117 from the whip element 106.
The helical element 117, which is covered with a dielectric 102, is fixed to the feeder 104 at its opposite side to the whip element 106. The helical element 117 is electrically disconnected from the whip element 106 with the separator 105.
The antenna 101 is attached slidably to the casing 120 with a conductive support 108. The support 108 is fixed to the casing 120 and is electrically connected to a specific radio circuit (not shown) provided in the casing 120.
When the whip element 106 is pulled out from the casing 120 (i.e., the antenna 101 is in the extended state), as shown in FIG. 1A, both the whip and helical elements 106 and 117 are located outside the casing 120 and at the same time, the stopper 109 is contacted with the support 108. In this state, the whip element 106 is electrically connected to the radio circuit provided in the casing 120 by way of the stopper 109 and the support 108, thereby feeding electric power to the element 106. Thus, the element 106 is activated and performs its operation.
On the other hand, when the whip element 106 is pushed into the casing 120 (i.e., the antenna 101 is in the retracted state), as shown in FIG. 1B, only the helical element 117 is located outside the casing 120 and at the same time, the feeder 104 is contacted with the support 108. In this state, the helical element 117 is electrically connected to the radio circuit provided in the casing 120 by way of the feeder 104 and the support 108, thereby feeding electric power to the element 117. Thus, the element 117 is activated and performs its operation.
As explained above, only the whip element 106 is activated when the antenna 101 is in the extended state while only the helical element 117 is activated in the retracted state. Therefore, impedance matching can be improved in each state. Thus, in recent years, the prior-art antenna 101 has been extensively used for portable radio devices such as cellular phones.
FIG. 2 shows a graph showing the relationship between the bandwidth of the prior-art antenna 101 and the length of the casing 120 in the retracted state. The curves of FIG. 2 indicate the bandwidth values where the return loss is equal to or less than xe2x88x9210 dB. The curves were obtained by numeric calculation under the condition that the resonance frequency of the helical element 117 was set as 800 MHz and 1.5 GHz without changing the shape and size of the element 117.
As seen from FIG. 2, the bandwidth varies as the casing length changes when the operating frequency is 800 MHz. This means that there is a value of the casing length that maximizes the bandwidth. Unlike this, when the operating frequency is 1.5 GHz, the bandwidth is kept approximately constant in spite of change of the casing length. Thus, it is unable to be said that there is a value of the casing length that maximizes the bandwidth. Also, as seen from FIG. 2, the bandwidth for 1.5 GHz is as low as approximately equal to half (xc2xd) to one-fifth (⅕) the bandwidth for the 800 MHz.
As described above, with the prior-art antenna 101 shown in FIGS. 1A and 1B, there is a problem that the obtainable bandwidth for the operating frequency of 1.5 GHz is not as wide as desired in the retracted state. In other words, satisfactory impedance matching is not implemented with respect to the helical element 117 that is activated in the retracted state.
Accordingly, an object of the present invention is to provide a retractable/extendable antenna for portable radio device that expands the bandwidth for the operating frequency of 1.5 GHz.
Another object of the present invention is to provide a retractable/extendable antenna for portable radio device that makes it possible to realize satisfactory impedance matching in both the extended and retracted states.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, a retractable/extendable antenna for portable radio device is provided, which is operable in an extended state and a retracted state. The antenna comprises:
(a) a casing;
(b) a first antenna element attached retractably or extendably to the casing;
the first element being formed by a linear antenna element;
the first element being located outside the casing and fed with electric power to be active in the extended state;
the first element being located inside the casing and fed with no electric power to be inactive in the retracted state; and
(c) a second antenna element connected mechanically to one end of the first element and disconnected electrically therefrom;
the second element being formed by a linear antenna element and shorter than the first element;
the second element being located outside the casing and fed with no electric power to be inactive in the extended state;
the second element being located outside the casing and fed with electric power to be active in the retracted state.
With the retractable/extendable antenna according to the first aspect of the invention, the first antenna element is attached retractably or extendably to the casing and at the same time, the second antenna element is connected mechanically to one end of the first element and disconnected electrically therefrom. The first element is formed by a linear antenna element (e.g., a whip element) while the second element is formed by a linear antenna element (e.g., a rod-shaped element) shorter than the first element.
Moreover, the first element is located outside the casing and fed with electric power to be active in the extended state. The first element is located inside the casing and fed with no electric power to be inactive in the retracted state. The second element is located outside the casing and fed with no electric power to be inactive in the extended state. The second element is located outside the casing and fed with electric power to be active in the retracted state.
Accordingly, in the retracted state where the second element is fed with electric power to be active and the first element is inactive, the bandwidth varies conspicuously with the change of the length of the casing at the operating frequency of 1.5 GHz in a similar way to that at the operating frequency of 800 MHz. This means that there is a value of the length of the casing that maximizes the bandwidth in the retracted state even when the operating frequency is 1.5 GHz. As a result, the bandwidth for the operating frequency of 1.5 GHz can be expanded.
For example, the bandwidth for the operating frequency of 1.5 GHz can be expanded to twice as wide as that of the prior-art antenna 101 or greater.
On the other hand, if a proper reactance element is added to the first element by some means, the input impedance of the antenna in the extended state is changed. If another proper reactance element is added to the second element by some means, the input impedance of the antenna in the retracted state is changed Thus, the values of the input impedance of the antenna in the extended and retracted states can be made closer or can be approximately equalized. As a result, satisfactory impedance matching can be realized in both the extended and retracted states.
In a preferred embodiment of the antenna according to the first aspect of the invention, a conductive support fixed to the casing is additionally provided. The first element is contacted with the support in the extended state and fed with electric power by way of the support. The second element is contacted with the support in the retracted state and fed with electric power by way of the support. The support, the first element, and an intervening dielectric constitute a first adjusting capacitor in the extended state. The support, the second element, and an intervening dielectric constitute a second adjusting capacitor in the retracted state.
In another preferred embodiment of the antenna according to the first aspect of the invention, a conductive stopper fixed to bottom of the first element by way of a dielectric is additionally provided. The first element, the stopper, and the intervening dielectric constitute an adjusting capacitor in the extended state.
In still another preferred embodiment of the antenna according to the first aspect of the invention, a dielectric separator is additionally provided to mechanically connect the first element to the second element and electrically disconnect the first element from the second element. The second element, the casing, and the intervening separator constitute an adjusting capacitor in the retracted state.
In a further preferred embodiment of the antenna according to the first aspect of the invention, the first element is designed for being electrically connected to a terminal matching circuit in the retracted state. The terminal matching circuit provides an adjusting reactance element in the retracted state. In this embodiment, there is an additional advantage that an inductor can be realized as the adjusting reactance element.
In this case, it is preferred that a dielectric separator is additionally provided to mechanically connect the first element to the second element and electrically disconnect the first element from the second element The second element, the casing, and the intervening separator constitute an adjusting capacitor in the retracted state. In this embodiment, there is an additional advantage that an inductor can be realized as the adjusting reactance element for the first element in the extended state while the adjusting capacitance is added for the second element in the retracted state. This facilitates improvement of impedance matching in both the extended and retracted states.
In a still further preferred embodiment of the antenna according to the first aspect of the invention, a dielectric stopper fixed to the first element and a conductive piece fixed to a surface of the stopper are additionally provided. The first element, the piece, and the intervening stopper constitute an adjusting capacitor in the extended state.
According to a second aspect of the present invention, another retractable/extendable antenna for portable radio device is provided, which is operable in an extended state and a retracted state. The antenna comprises:
(a) a casing;
(b) a first antenna element attached retractably or extendably to the casing;
the first element being formed by a linear antenna element;
the first element being located outside the casing and fed with electric power to be active in the extended state;
the first element being located inside the casing and inactive in the retracted state; and
(c) a second antenna element connected mechanically to one end of the first element and connected electrically to the first element;
the second element being formed by a linear antenna element and shorter than the first element;
the second element being located outside the casing and fed with electric power to be active in the extended state;
the second element being located outside the casing and fed with electric power to be active in the retracted state.
With the retractable/extendable antenna according to the second aspect of the invention, the first antenna element is attached retractably or extendably to the casing and at the same time, the second antenna element is connected mechanically to one end of the first element and connected electrically thereto. The first element is formed by a linear antenna element (e.g., a whip element) while the second element is formed by a linear antenna element (e.g., a rod-shaped element) shorter than the first element.
Moreover, the first element is located outside the casing and fed with electric power to be active in the extended state. The first element is located inside the casing and fed with no electric power to be inactive in the retracted state also. The second element is located outside the casing and fed with electric power to be active in the extended state. The second element is located outside the casing and fed with electric power to be active in the retracted state.
Accordingly, in the retracted state where only the second element is fed with electric power to be active, the bandwidth varies conspicuously with the change of the length of the casing at the operating frequency of 1.5 GHz in a similar way to that at the operating frequency of 800 MHz. This means that there is a value of the length of the casing that maximizes the bandwidth in the retracted state even when the operating frequency is 1.5 GHz. As a result, the bandwidth for the operating frequency of 1.5 GHz can be expanded.
For example, like the above-described antenna of the first aspect, the bandwidth for the operating frequency of 1.5 GHz can be expanded to twice as wide as that of the prior-art antenna 101 or greater.
On the other hand, if a proper impedance or reactance element is added to the combination of the first and second elements by some means, the input impedance of the antenna in the extended state is changed. If another proper impedance or reactance element is added to the combination of the first and second elements by some means, the input impedance of the antenna in the retracted state is changed Thus, the values of the input impedance of the antenna in the extended and retracted states can be made closer or can be approximately equalized. As a result, satisfactory impedance matching can be realized in both the extended and retracted states.
In a preferred embodiment of the antenna according to the second aspect of the invention, the first element is integrated with the second element.
In another preferred embodiment of the antenna according to the second aspect of the invention, the first element is electrically connected to the second element by way of a conductive member. The first and second elements are fed with electric power by way of the member in the retracted state.
In still another preferred embodiment of the antenna according to the second aspect of the invention, a conductive support fixed to the casing is additionally provided. The first element is contacted with the support in the extended state and fed with electric power by way of the support. The second element is contacted with the support in the retracted state and fed with electric power by way of the support. The support, the second element, and an intervening dielectric constitute an adjusting capacitor in the retracted state.
In a further preferred embodiment of the antenna according to the second aspect of the invention, a conductive stopper fixed to bottom of the first element by way of a dielectric is additionally provided. The first element, the stopper, and the intervening dielectric constitute an adjusting capacitor in the extended state.
In a still further preferred embodiment of the antenna according to the second aspect of the invention, the first element is designed for being electrically connected to a terminal matching circuit in the retracted state. The terminal matching circuit provides an adjusting reactance element in the retracted state. In this embodiment, there is an additional advantage that an inductor can be realized as the adjusting reactance element.
In a more further preferred embodiment of the antenna according to the second aspect of the invention, a dielectric stopper fixed to the first element and a conductive piece fixed to a surface of the stopper are additionally provided. The first element, the piece, and the intervening stopper constitute an adjusting capacitor in the retracted state.